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So-called skeptics usually claim that we don't know some normal facts because they know some abnormal fact about some possibility that undermines knowledge. How do these people know their modal claims are true? For example, we wouldn't know that "it is possible for a brain in a vat to have experiences indistinguishable from normal ones" unless we actually knew that neuroscience was true, which would require prior access to the real world.

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    Skeptics do not claim to know abnormal facts, or any, they doubt everything and challenge those who do not to justify themselves. One does not need to know whether neuroscience is correct to use it in an objection, accepting a premise of the opponent is the usual move for a reductio. The goal of a skeptic is not to justify anything, but to undermine justifications offered by others.
    – Conifold
    Nov 28, 2019 at 22:30
  • As the Stanford Encyclopedia of Philosophy article on skepticism points out, there is a difference between so-called academic or Cartesian skepticism, which is exactly what I am addressing, whereas for some reason all of you are bringing up the alternative form of skepticism. Nov 28, 2019 at 23:14
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    This is why it helps to be specific and provide references, if you want pointed answers ask pointed questions. Cartesian skeptics postulate knowability of meta-propositions about powers of reason and judgment, so they are entitled to any possibility claims that the reason judges coherent. They may even accept that such claims are defeasible, but the burden of defeasing them is on the opponent. And, of course, they are as free to set up reductios for opponents' positions as Pyrrhonians, or anybody else.
    – Conifold
    Nov 29, 2019 at 1:14
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    – J D
    Nov 29, 2019 at 14:15

2 Answers 2

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The skeptic's claim is not "We are certain that we could be brains in vats" - rather, it's "We are not certain that we are not brains in vats."

This claim is fundamentally different. To defeat it, one would have to provide an airtight proof that the brains-in-vats scenario is impossible, and it's hard to see how to even get started here. In fact, this is part of the skeptic's broader point: that this deep skepticism is more-or-less impossible to refute once considered seriously, while still being enough to counter strong certainty claims (e.g. my "certainty" that I have two hands is based on information from my senses, which - per the skeptic - I am not truly certain is not false).

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  • How would anyone ever know that it was impossible to prove or disprove? How can the skeptic even attempt to argue with someone who directly knows there is an external world? Which everyone arguing on a public forum should know. Nov 28, 2019 at 23:03
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    @KristianBerry How can anyone directly know there is an external world?
    – user68014
    Nov 29, 2019 at 9:54
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    @KristianBerry - It's not rocket-science. You cannot design a test to prove you are not a brain in a vat. Ergo you might be one. I don't know what the fuss is about since most people already believe they are a brain in a skull.
    – user20253
    Nov 29, 2019 at 14:23
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    @urhen - Yes, what follows is that it might be possible. .
    – user20253
    Nov 30, 2019 at 14:01
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    @urhen - Struggling to see your issue here. If something might be the case then it might be the case. No need for a change of language. .
    – user20253
    Dec 1, 2019 at 14:03
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You don't have to be certain that it's possible to be a brain in a vat for the argument to work. You just have to not be certain that it's impossible to be one.

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  • How does the skeptic know that they don't have to be certain for the argument to go through? Nov 28, 2019 at 23:03
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    @KristianBerry As formulated by Déscartes (which is probably the best known formulation) the procedure is to doubt everything you aren't sure of. It's not to doubt only what you're sure you're not sure of, which would kind of deadlock it from the start. But I actually think your criticism is valid. The best way to attack GS is to look at the assumptions it makes, especially as it claims not to make any.
    – user68014
    Nov 29, 2019 at 9:53

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